Down the Drain! — Heat Recovery for Your Hot Water

Drainwater Heat Recovery Improves Water Heating Efficiency

Do you use hot water in your home? I thought so. You have a water heater then, right? You know, one of those things that you pump money into all the time, perhaps to the tune of $300 or $400 per year, so you don’t freeze when you take a shower.

And of course, you use every BTU of heat you put into that water, don’t you? What?! No? You mean, you’re sending hot water—and money—down the drain?

Well, you’re not alone. Most people do the same thing, including me. You pay your hard-earned money to run that water heater, use that hot water oh so briefly, and then send all those BTUs that you paid to put into the water right down the drain. Where they keep the sewer lines or septic tanks warm and fertile.

There’s an alternative, though, and it’s starting to get some traction. The photo above shows a drain-water heat recovery system that I saw at a home in Ontario. It’s basically a copper pipe wrapped around the drain pipe, where it can absorb a lot of the heat that’s in the drain water. That heat goes into the water that enters your water heater, as shown in the diagram below.

drain water heat recovery diagram energy factor

When you’re using hot water in a system without drain-water heat recovery, cold water from your municipal water supply or well is continuously running into your water heater.

(Brief rant: Because of this, it really should be called a ‘cold water heater,’ not a ‘hot water heater.’ I just call it a water heater myself. Rant over.)

With a drain-water heat recovery system, the cold water heading to the water heater first picks up some of that waste heat from the drain pipe. Thus, it enters the water heater pre-warmed and will take less heat energy—and money—to get it up to temperature. The efficiency of your water heating system goes up, and your water heating bill goes down!

For you energy modelers and HERS raters out there, REM/Rate doesn’t have a way that I know of to include drain-water heat recovery directly, but the Hot2000/EnerGuide raters I taught in Toronto last week are adding 0.12 to the energy factor (EF) of whatever water heating EF they’re starting with. I don’t have a source for where the 0.12 EF comes from, though. Anyone have more info on this?

 

Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog and is writing a book. You can follow him on Twitter at @EnergyVanguard.

 

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How to Install a Branched-Drain Greywater System in a Green Home

 

Drain-water heat recovery diagram from the Energy Savers website at the US Department of Energy.

 

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This Post Has 22 Comments

  1. I can see a lot of use for
    I can see a lot of use for this in industrial processes that use a lot of hot water. Perhaps there is already a lot of this in use already in industrial processes. 
     
    It would be great to have a couple of case studies.

  2. Seems to me that the design
    Seems to me that the design in your schematic could be made even more efficient if the heat-exchanger coil was inside the drain pipe, or at least somehow configured so that the coil got fully immersed by the hot waste water, rather than just being wrapped around the outside of the waste pipe.  
    It’s interesting to me, from an engineering perspective, how we often end up designing “bolt on” sorts of solutions to route waste heat back into the plant. Like turbo chargers, condensing furnaces/boilers, etc. I often think that if any heat-energy-oriented system were designed the right way in the first place, there wouldn’t be so much need to invent all these bolt-on recovery mechanisms after the fact…

  3. Poole has a good comment.
    Poole has a good comment. All of the large energy systems would benefit from being an integrated system rather than separate units. HVAC, water heater, washer/dryer, dish washer… all integrated and directing recovered energy to whatever purpose it is needed. No need to limit recovered water heat to heating water. It could be warming (or cooling) the house, or even generating electricity. Such a system would also be easier to measure as to efficiency.

  4. Chris C.:
    Chris C.: I’ve posted this on the RESNET BPI group on LinkedIn, so maybe someone there will have some good resources. 
     
    John P.: That’s certainly an interesting way to look at it. I’m not sure an in-pipe system would be worth it, though. Water going down the drain pipe is all on the surface of the pipe, not going through the middle, so there’s actually pretty good heat transmission ability with the recovery pipe on the outside. I do agree that more systems thinking overall is in order, however.

  5. John M.:
    John M.: Yes, integration is a good thing. Achieving it is another matter. Turning waste heat into electricity, for example, would hardly be cost effective because there’s just not enough energy to do it effectively. Heat engine efficiency scales with the temperature difference between the source and the sink, and with waste heat in a home, that difference would be low, resulting in low efficiency conversion. Another drawback is that trade contractors often don’t work well with other trades in the construction process. That would require some serious restructuring. Of course, we need some serious restructuring because the system we have now doesn’t work well.

  7. Enviroman:
    Enviroman: Yeah, I’ve seen them in the LEED and EarthCraft House rating systems for years now, but I think Saturday in Ontario was the first time I’ve actually seen one installed. I haven’t really investigated their achieved energy efficiency yet, so I don’t know how cost effective they are. If they do boost the EF by 0.12, I’d think they might be. Then again, copper ain’t cheap these days, and it’s going to depend on how much hot water a family uses.

  8. I considered a GFX when I
    I considered a GFX when I built my previous home in 1999. At the time, gas was $0.67/therm and I just couldn’t convince myself that the savings would be enough to justify the price. Now that gas prices are a bit higher, maybe, but GFX price has also gone up with the price of copper. 
     
    A home with two showers feeding a single drain pipe riser nearby, and 3 or 4 people taking daily showers would be more likely to see a payback. 
     
    Allison: One thing you failed to point out is that heat exchange only takes place if the drain water happens simultaneously with the supply water. A shower would work, but not a dishwasher, clothes washer or bath tub. Your drawing is a little misleading in that regard 🙂

  9. Natural Resources Canada’s
    Natural Resources Canada’s Office of Energy Efficiency as of March 2011 released an energy credit table (used for Hot2000 modeling in the EnerGuide system) that lists a kWh/yr credit that can be applied to a home based on the model of DWHR installed, as well as the # of showers draining through it. It’s a bit more precise than the +0.12 add-on to the EF. Not sure where (if at all) they link it on their site directly, but most CAN energy advisers would have access to it.

  10. Great idea, now if there was
    Great idea, now if there was a way to get it to work on a slab construction house. How does payback time compare to solar hot water heating?

  12. The heat exchange takes place
    The heat exchange takes place even if there is no water draw. The fresh water “resting” in the supply line spiral will be heated whether it is moving or not. Batch processes like dishwashers, clothes washers would work, just at a lower efficiency. Tub baths would require some later hot water usage to take advantage of the heat, otherwise it would be lost. 
     
    The drawing isn’t at fault, BTW for any misunderstanding.

  13. There’s an alternative to
    There’s an alternative to this one. Some 20,000 homes in Europe now have it installed!We do have case studies for commercial applications also. Please do contact me to get them. Looking for distributors in US if you have any ideas?

  14. What about on-demand water
    What about on-demand water heaters. Would your really add the 0.12 to the EF?

  15. Good point by David Butler.
    Good point by David Butler. An optimization might be, when hot waste water isn’t actually flowing, hold some quantity of it in an insulated holding tank. The supply coils could be immersed in the water in the tank. Then, when the held water drops below some optimum temperature (what ever that is), or more hot waste water starts to come in, release the old water. 🙂

  16. David B.:
    David B.: Thanks for your comment about your analysis on this for your previous home. I think the price of copper may limit the cost-effectiveness of these devices except in homes where people take a lot of showers. And yes, I neglected to mention that in/out simultaneity issue. Thanks for raising that point, but even when when the water heater’s not drawing water in while the hot water goes down the drain, some of that heat may be recovered the next time someone runs hot water, unless it’s a long time later. 
     
    Kyle A.: Do you list kWh/yr just as a general energy unit? Do they give the number in Btu/yr or GigaJoules/yr or m^3/yr if it’s a gas water heater? I agree that the 0.12 added on to the EF is a bit imprecise. 
     
    Bob: That’s a good question. Once I find out more about the numbers, I’ll let you know here. 
     
    Robert D.: I’m with you on that one. Most homes have huge inefficiencies in hot water distribution and waste a lot of energy and water. Easy to fix in new homes with good design. Not so easy in existing homes. 
     
    Donald B.: Yes, thanks for pointing out that some of the heat still gets recovered even when the hot water’s not running when hot water’s going down the drain. 
     
    Tony G.: From your website, it looks like the one you’re selling is the same as I described above, right? 
     
    Josh L.: That’s what the raters in my class in Toronto told me they’ve been instructed to do. 
     
    John P.: Yeah, that could work. Now, can you make it cheaply enough to be cost effective? Will it take another tank? Where will it go?

  17. wow, add 0.12 to your EF?
    wow, add 0.12 to your EF? that seems extremely generous. it is also a function of the efficiency of your heater, as some other commenters have noted indirectly.  
     
    interesting study here, with calculations to estimate savings: http://www.regie-energie.qc.ca/audiences/3637-07_2/DDR3637_2/RepDDR/B-12-GI-23Doc1-2_RepDDRSE-AQLPA_3637-2_28sept07.pdf 
     
    re: simultaneity, the study states: It was concluded that DWHR devices only recovered energy during simultaneous water draws (showers) and that for modelling purposes all other water draws could be ignored.

  18. Mike:
    Mike: Thanks for the link! I’ll check it out. I think I agree that 0.12 additional in the EF seems high. For modeling purposes, it’s best to ignore the non-simultaneous uses to err on the conservative side. The actual use, then, would be better than modeled if any of the non-simultaneous heat is captured.

  19. Great idea. I have a tank
    Great idea. I have a tank which collects all the water from the bath,washing machine etc. and recycles it through the the toilets( info@hldbc.ca). All the heat stays in the house and saves water.I have a sewer through my property and have always wanted to put a ground source pump on it. I was told this has been tried and the sewer froze-yikes! I guess the snow will continue to melt off the manhole covers. Cheers Mike Legge

  21. Anybody ever compared the
    Anybody ever compared the cost/savings of the GFX vs. simply insulating the hot water lines throughout the home? I’m all for reclaiming the heat lost down the drain, but the GFX looks less than optimal in most slab-on-grade construction and most retrofits. What about the extra plumbing material/cost required to run additional “cold” water lines to transport the recaptured heat. Then you have heat lost along those lines before making it back to the WH. The longer those lines, the less sense this solution makes, and the more potential for a plumbing leak/issue down the road. “Keep it Simple, Stupid”, as the saying goes….

  22. To get a better idea on the
    To get a better idea on the savings potential for drain water heat recovery technology – you should try this calculator. The ThermoDrain TD342B is equivalent to the G3-40 product in efficiency. 
     
    http://www.ceati.com/calculator/ 
     
    We manufacturer the ThermoDrain and send our customers to this website to get a better idea on what the savings would look like for their particular application. Each application has several variables and this calculator takes all in consideration including the temperature of the ground water. 
     
    Daniel Beauchemin 
    EcoInnovation Technologies 
    http://www.ecoinnovation.ca 
    888-881-7693 

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